The use of beams of neutrons to interrogate and locate substances at a distance is an emerging technology. As disclosed in the applicant's co-pending U.S. patent application Ser. No. 12/503,300, Filed: Jul. 15, 2009, the entire disclosure of which is hereby incorporated by reference and relied upon, a source of thermal, epithermal, or cold neutrons can be used to interrogate an Area Under Investigation (AUI) at a distance, and then detect and analyze gamma rays returning from the AUI in order to identify substance of interest in the AUI or its immediate surroundings.
Neutrons sent to interrogate an AUI will produce a broad range of reactions both in the AUI and also in virtually all other substances that are exposed to them, including, but not limited to: materials in the neutron source itself, including but not limited to shielding and the materials used to produce the neutrons; the intervening atmosphere with all its constituents; materials surrounding the AUI in all directions, including objects beside, in front of, and behind the AUI; and uninteresting substances commingled with the AUI. Signals resulting from such materials other than the AUI are referred to as “background signals” or, alternatively “nuisance signals”.
Gamma rays resulting from neutron reactions are more readily detected at a distance than are other rays or nuclear particles resulting from those reactions.
The magnitude of the gamma ray reaction flux from each substance varies with the magnitude of the neutron flux delivered to that substance.
Both the neutron beam sent to interrogate the AUI and the returning gamma ray flux from the AUI and also the intervening and neighboring substances obey the so-called “1/r2 rule” (“1-over-r-squared rule”), namely, that the flux density of each varies as the square of the distance between the radiation center and the point at which the flux is measured.
The combined effect of the 1/r2 rule on both the interrogating neutron beam and the returning gamma rays is the so-called “radar equation”, or “1/r4 rule” (“1-over-r-to-the-fourth rule”)—returning gamma ray flux resulting from a neutron beam interrogation of an AUI and its surroundings varies as the fourth power of distance from each.
A final effect is the attenuation of both the interrogating neutron beam and the returning gamma ray signal. Such attenuation is exponential—both beams are attenuated by intervening air and other intervening substances, at an exponential rate.
The net effect of all these individual effects is that all signals returned to the detector vary strongly with the distance to the AUI and its surroundings.
Signals entering the detector are the sum of signals produced by a substance or substances of interest, if any are present, plus signals due to nuisance sources, artifacts, and background, collectively referred to as “Noise”. The challenge for all detection systems of any type is the separation of signals due to items or substances of interest from signals due to noise. A key strategy for detection is the relative reduction of signals due to noise when compared to signals of interest. The ratio of total signal to signals due to noise is known as the “Signal-to-Noise Ratio” (SNR).
Since the statistical characteristics of signals of interest and signals due to noise are different from one another, effective separation of signals of interest from noise can be achieved by accumulating the total signal for a period of time, known as the Integration Time or Sampling Time (Δt), and then analyzing the accumulated signal. In general, a higher SNR allows a shorter Integration Time for the same level of detection confidence. Since Integration Time is virtually always critical, increasing SNR is always a desirable goal of a detection system, since it tends to reduce critical Integration Time.
In neutron gamma fluorescence detection systems, over a very broad range, signals returning to the detector from substances of interest scale directly with the flux of neutrons used to illuminate them, for a given distance to the AUI.
However, not all noise signals present in neutron fluorescence detection systems scale in the same way as signals from substances of interest. An entire significant class of noise signals, known as either “pulse pile up” or “random summing” noise, scales disproportionately with respect to illuminating neutron flux, at any given distance to the AUI, when compared to signals from substances of interest. When combined with the very large changes in total received signal with relatively small changes in distance to the AUI, the result of this behavior is that even small changes distance to the AUI can change SNR dramatically by changing “pulse pile up” or “random summing” noise faster than signals of interest change.
“Pulse pile up” or “random summing” noise at any given distance to an AUI can be adjusted for the best SNR and Integration Time by adjusting the illuminating neutron flux.
Therefore, there exists a need in the art to enable the agile adjustment of illuminating neutron flux with respect to the measured distance to the AUI, as well as with respect to other parameters, in order to allow optimization of SNR an Integration Time.